1. Field of the Invention
The present invention relates generally to a fuel injection system for an engine, and more specifically, to a common-rail fuel injection system for a diesel engine.
2. Description of the Prior Art
Common-rail fuel injection systems have been known as disclosed in such as Japanese First (unexamined) Patent Publication No. 59-165858 and U.S. Pat. No. 4,545,352 which is an equivalent of the former. In the common-rail fuel injection systems, high pressure fuel is accumulated in a so-called common rail working as a surge tank to be injected into engine cylinders via opening and closing operations of respective fuel injectors.
As shown in FIG. 1, a common-rail fuel injection device 100 of this type includes an injection nozzle 101 through which the high pressure fuel from the common rail is injected into the corresponding engine cylinder, and a three-way solenoid valve 102 which controls a fuel injection timing and a fuel injection amount.
The injection nozzle 101 includes a nozzle needle 103 operative to open and close injection holes, a hydraulic piston 104 operative to drive the nozzle needle 103, and a control chamber 105 operative to control a hydraulic pressure to be applied to the hydraulic piston 104. As shown in FIG. 2, a pressure control valve 107 is provided in the control chamber 105. The pressure control valve 107 is formed with an orifice 109 extending through the pressure control valve 107 at its center. A reference numeral 108 denotes a portion of the three-way solenoid valve 102, defining a communication passage 106 and working as a valve seat for the pressure control valve 107.
Practically, the orifice 109 only works to control the flow of the hydraulic pressure from the control chamber 105 into the communication passage 106 of the three-way solenoid valve 102 as will be clear from the following explanation with reference to FIG. 3.
FIG. 3 is a timechart showing a relationship among a hydraulic pressure in the control chamber 105, a lift position of the nozzle needle 103 and a load applied to a value seat for the nozzle needle 103.
At the start of the fuel injection, which corresponds to FIG. 2, the three-way solenoid valve 102 allows the communication passage 106 to communicate with a low pressure side. Accordingly, the pressure control valve 107 is seated on the valve seat 108 to allow the high pressure fuel within the control chamber 105 to slowly flow out via the orifice 109 in a controlled fashion, as shown in part (A) of the graph in FIG. 3. When the hydraulic pressure in the control chamber 105 drops to a value opening pressure for the nozzle needle 103, the hydraulic piston 104 starts to slowly go up resulting in lifting up the nozzle needle 103 as shown in part (B) of the graph in FIG. 3. This means that the nozzle needle 103 starts to separate from its valve seat in a nozzle body 110 to allow the start of the fuel injection via the injection holes into the corresponding engine cylinder.
On the other hand, at the end of the fuel injection, the three-way solenoid valve 102 allows the communication passage 106 to communicate with a high pressure side, i.e. the common rail. Accordingly, the high pressure fuel is applied to the pressure control valve 107 to urge the same toward the hydraulic piston 104. Thus, the pressure control valve 107 is separated from the valve seat 108 to allow immediate introduction of the high pressure fuel into the control chamber 105 via an annular gap formed between the outer periphery of the pressure control valve 107 and the peripheral wall of the control chamber 105. Accordingly, in this case, the orifice 109 does not function to control the flow of the orifice 109 does not function to control the flow of the high pressure fuel from the communication passage 106 into the control chamber 105. As a result, as shown in part (A) of the graph in FIG. 3, the pressure in the control chamber 105 immediately increases to a valve closing pressure for the nozzle needle 103. This leads to a quick overall downward movement of the hydraulic piston 104 to force the nozzle needle 103 onto the valve seat in the nozzle body 110.
With the foregoing structure, the prior art common-rail fuel injection systems are capable of providing the desirable so-called delta type fuel injection characteristics, that is, the fuel injection rate is small at the start of the injection and gradually gets larger, while, the sharp cut-off of the fuel injection is attained at the end of the injection.
The prior art common-rail injection systems, however, have the following problems.
As described above, the high pressure fuel is immediately introduced into the control chamber 105 at the end of the fuel injection. Accordingly, as shown in part (A) of the graph in FIG. 3, the hydraulic pressure in the control chamber 105 inevitably becomes overshot so that the nozzle needle 103 is forced down to a level exceeding a position of the nozzle needle 103 at the start of the fuel injection, as shown in part (B) of the graph in FIG. 3. This causes disadvantages that an excessive impact load P={(upper peak value)-(lower peak value)} is applied to the valve seat for the nozzle needle 103, as shown in part (c) of the graph in FIG. 3.
This necessitates associated portions around the valve seat in the nozzle body 110 to be made thicker so as to provide strength large enough against the applied impact load P. Mere provision of the larger thickness around the valve seat, however, inevitably increases a length of each injection hole so that an increased resistance against the flow of the injected fuel is resulted. On the other hand, in order to avoid such an increased resistance with the increased thickness, a volume of a sack chamber 111 should be enlarged. This, however, causes the following problems.
The sack chamber 111 is located downstream of the valve seat for the nozzle needle 103 and is formed with the injection holes at its downstream end portions. Accordingly, the fuel in the sack chamber 111 is likely to flow out into the corresponding engine cylinder via the injection holes even after the completion of the fuel injection, i.e. even after the nozzle needle 103 is seated on the valve seat. This means that the enlarged volume of the sack chamber 111 may lead to serious disadvantages such as increases of fuel consumption rate, exhaust gas temperature and hydrocarbon. In the circumstances, enlarging the thickness around the valve seat cannot be taken as measures for solving the problem of the excessive impact load P in view of the other serious problems caused therefrom.